Assume the following history exists and the current branch is "topic":

                     A---B---C topic
                    /
               D---E---F---G master

       From this point, the result of either of the following commands:

           git rebase master
           git rebase master topic

       would be:

                             A'--B'--C' topic
                            /
               D---E---F---G master

Here is how you would transplant a topic branch based on one branch to another, to pretend that you forked the topic branch from the latter branch, using rebase --onto.

       First let’s assume your topic is based on branch next. For example, a feature developed in topic depends on some functionality which is found in next.

               o---o---o---o---o  master
                    \
                     o---o---o---o---o  next
                                      \
                                       o---o---o  topic

       We want to make topic forked from branch master; for example, because the functionality on which topic depends was merged into the more stable master branch. We want our tree to look like this:

               o---o---o---o---o  master
                   |            \
                   |             o'--o'--o'  topic
                    \
                     o---o---o---o---o  next

       We can get this using the following command:

           git rebase --onto master next topic

git remote add gh <initial_repo>
git remote add origin <my_repo>
git pull gh